The structures of a group of newly and independently isolated defective substituted variants of SV40 were determined. These variants were initially detected by screening for variants containing monkey DNA sequences homologous to a monkey DNA segment found in a previously characterized variant. Five independent new isolates proved to be essentially identical to the variant obtained previously. The results suggest that SV40 variants do not arise totally at random but rather that certain structures recur. The close similarity between these structures suggests that some mechanism other than recombination between the monkey genome and the genome of the infecting virus may be involved in their formation. A recombinant "library" of monkey (C. aethiops) DNA in bacteriophage lambda has been constructed and is being used to investigate those monkey DNA segments that occur in the defective SV40 variants.